1. Field of the Invention
The present invention relates to a magnetic recording medium having a high recording capacity, a high access rate and a high transmission rate, in particular, a magnetic recording medium for data backup.
2. Prior Art
Magnetic tapes find various applications such as audio tapes, video tapes, computer tapes, etc. In particular, in the field of tapes for data backup, with the increase of the capacity of a hard disc which should be backed up, a tape having a memory capacity of several ten GB per one volume has been commercialized, and it is inevitable to increase the capacity of the backup tape to cope with the further increase of the capacity of the hard disc. Furthermore, it is necessary to increase a relative speed between the tape and a magnetic head to increase the access rate and the transmission rate.
With the magnetic tape which can cope with the increase of the memory capacity per one volume and the increase of the travelling speed of the tape and the relative speed between the tape and the magnetic head, it is necessary to improve the touch between the tape and the magnetic head through the optimization of the mechanical properties of a non-magnetic support, a primer layer and a magnetic layer as well as the increase of a recording density through the improvement of the magnetic layer with the increase of the magnetic properties and dispersion of a ferromagnetic powder and the increase of the memory capacity through the increase of the tape length per one volume with the reduction of the total thickness of the tape.
In connection with the improvement of the magnetic properties of the ferromagnetic powder, a ferromagnetic metal ion powder is mainly used in place of conventionally used metal oxide powders or cobalt-containing iron oxide powder, since the larger residual magnetization in the magnetic layer is more preferable for the increase of output. Thus, a ferromagnetic iron-based metal powder having a coercive force of 120 A/m (1,500 Oe) or more is proposed (for example, JP-A-6-25702, JP-A-6-139553, etc.)
To improve the dispersion of the ferromagnetic powder, it is proposed to use a binder having a polar functional group such as a sulfonic acid group, a phosphoric acid group or its alkali salt, to use a low molecular weight dispersant together with a binder, to continuously carry out kneading and dispersing steps of a magnetic paint, or to add a lubricant to a magnetic paint after dispersing (for example, JP-A-2-101624, JP-A-3-216812, JP-A-3-17827, JP-A-8-235566, etc.)
To improve the touch between the tape and the magnetic head so as to decrease spacing loss between them, it is proposed to smoothen the magnetic layer under conditions of a high temperature and a high pressure in a calendering step in addition to the increase of the dispersibility of the magnetic powder (for example, JP-B-1-1297, JP-B-7-60504, JP-A-4-19815, etc.)
In addition to the improvement of the properties of the magnetic layer, it is proposed to decrease the thickness of the magnetic layer to 0.6 μm or less with the provision of a primer layer between a non-magnetic support and the magnetic layer to make the structure of the magnetic recording medium suitable for sort wavelength-recording (JP-A-5-234063). Such a magnetic recording medium has the primer layer to decrease self-demagnetization loss and reproduction loss due to the reduction of the thickness of the magnetic layer and also to suppress the deterioration of the travelling property and durability of the magnetic recording media due to the reduction of the thickness of the magnetic layer.
On the other hand, with the recent development of recording systems, it is tried to further decrease the recording wavelength. For example, the latest digital data storage systems use the shortest recording wavelength of 0.5 μm or less. In general, as the thickness of the magnetic layer increases, the filling amount of the magnetic powder per unit area increases, and thus the output increases. However, when a ratio of the thickness of the magnetic layer to the wavelength exceeds a certain value, a demagnetizing field increases and thus the output does not further increase. Therefore, the thickness of the magnetic layer should be about one third (⅓) of the shortest recording wavelength. Accordingly, with the above-described latest recording systems, the thickness of the magnetic layer is reduced to 0.3 μm or less, and also the flatness of the surface of the magnetic layer should be improved.
In the case of the recording systems having the large capacity, the tape-travelling speed and the relative speed between the tape and the magnetic head tend to be further increased since it is necessary to increase the access rate and the transmission rate. When the tape-travelling speed and the relative speed between the tape and the magnetic head are increased, the touch between the magnetic head and the magnetic tape becomes unstable and the output fluctuates between the entrance and the exit of a track.
To improve the flatness of the magnetic layer corresponding to the reduction of the recording wavelength, it is necessary to use a non-magnetic support having high surface smoothness. However, the non-magnetic support having the high surface smoothness is very expensive and the travelling of the non-magnetic support becomes unstable since it slips or sticks to a roll when a coating layer such as the primer layer is formed. Therefore, the productivity of the magnetic recording media deteriorates.
When the thickness of the primer layer is decreased to 1.5 μm or less to decrease the total thickness of the magnetic recording medium, the flatness of the surface of the primer layer becomes insufficient. When the magnetic layer is formed on such a primer layer by a wet-on-wet method, minute unevenness is formed at the interface between the primer layer and the magnetic layer. Such unevenness not only adversely affect the writing and reading properties of the tape but also generates edge weave at tape edges when a raw sheet of magnetic tapes is slit in a specific width. The edge weave adversely affects the tracking of the magnetic head and thus cause the fluctuation of the output. This phenomenon is remarkable when the cheap non-magnetic support which has good travelling properties in the course of coating and low surface flatness.
Accordingly, when the non-magnetic support with low surface flatness is used, it is highly desired for magnetic recording tapes to cope with the decrease of the recording wavelength through the improvement of the flatness of the surface of a magnetic recording layer, and also to decrease the fluctuation of the output through the suppression of the edge weave at the tape edges and the fluctuation of the output of the magnetic head between the entrance and the exit of the track through the improvement of the touch between the magnetic head and the magnetic tape.